"'Object reference not set to an instance of an object" When I Started App

I have got some Classes implements IHostedService.

EmailSendService:

public class EMailSendService : IHostedService, IDisposable
{
    private Timer _timer;
    private IMailAccountService _mailAccountService;
    private IMailTaskDAL _mailTaskDal;
    private LoggerServiceBase _loggerServiceBase;

    public EMailSendService(IMailAccountService mailAccountService, IMailTaskDAL mailTaskDal)
    {
        _mailAccountService = mailAccountService;
        _mailTaskDal = mailTaskDal;
        _loggerServiceBase = (LoggerServiceBase)Activator.CreateInstance(typeof(DatabaseLogger));
    }

    private void SendMail(object state)
    {
        var isAnyMailSended = false;
        var logDetail = new LogDetail
        {
            UserID = "",
            UserAgent = "",
            IP = "System",
            MethodName = "SENDMAILTASK",
            StackTrace = "",
            TraceIdentifier = ""
        };

        try
        {
            var now = DateTime.Now;
            var mailTasks = _mailTaskDal.GetList(op => !op.IsSended && op.NextSendDate < now && op.TryCount < 10);
            isAnyMailSended = mailTasks.Count > 0;
            foreach (var mailTask in mailTasks)
            {
                var toList = mailTask.ToList.Split(',');
                var sendResult = _mailAccountService.SendMail(toList, mailTask.Subject, mailTask.Body);

                mailTask.Result = sendResult.Messages[0].Description;

                if (sendResult.Success)
                {
                    mailTask.IsSended = true;
                    mailTask.SendDate = DateTime.Now;
                }
                else
                {
                    mailTask.NextSendDate = DateTime.Now.AddMinutes(5);
                    mailTask.TryCount++;
                }

                _mailTaskDal.Update(mailTask);
            }
        }
        catch (Exception e)
        {
            logDetail.ExceptionMessage = e.Message;
            logDetail.StackTrace = e.StackTrace;
        }
        finally
        {
            if (isAnyMailSended)
                _loggerServiceBase.Info(logDetail);
        }
    }

    public System.Threading.Tasks.Task StartAsync(CancellationToken cancellationToken)
    {
        _timer = new Timer(SendMail, null, TimeSpan.Zero, TimeSpan.FromSeconds(30));
        return System.Threading.Tasks.Task.CompletedTask;
    }

    public System.Threading.Tasks.Task StopAsync(CancellationToken cancellationToken)
    {
        _timer?.Change(Timeout.Infinite, 0);
        return System.Threading.Tasks.Task.CompletedTask;
    }

    public void Dispose()
    {
        _timer?.Dispose();
    }
}

CreateActivityTask:

public class CreateActivityTask : IHostedService, IDisposable
{
    private Timer _timer;
    private LoggerServiceBase _loggerServiceBase;

    public CreateActivityTask()
    {
        _loggerServiceBase = (LoggerServiceBase)Activator.CreateInstance(typeof(DatabaseLogger));
    }

    private void CreateActivity(object state)
    {
        var logDetail = new LogDetail
        {
            UserID = "",
            UserAgent = "",
            IP = "System",
            MethodName = "CreateActivityTask",
            StackTrace = "",
            TraceIdentifier = ""
        };

        try
        {
            //create activity
        }
        catch (Exception e)
        {
            logDetail.ExceptionMessage = e.Message;
            logDetail.StackTrace = e.StackTrace;
        }
        finally
        {
            _loggerServiceBase?.Info(logDetail);
        }
    }

    public System.Threading.Tasks.Task StartAsync(CancellationToken cancellationToken)
    {
        _timer = new Timer(CreateActivity, null, TimeSpan.Zero, TimeSpan.FromSeconds(60));
        return System.Threading.Tasks.Task.CompletedTask;
    }

    public System.Threading.Tasks.Task StopAsync(CancellationToken cancellationToken)
    {
        _timer?.Change(Timeout.Infinite, 0);
        return System.Threading.Tasks.Task.CompletedTask;
    }

    public void Dispose()
    {
        _timer?.Dispose();
    }
}

I Use two extension function to add this classes in startup.cs -> ConfigureServices:

 services.AddHostedService<EMailSendService>();
 services.AddHostedService<CreateActivityTask>();

But if i use like that, program does not run. When i start it, this code throws an error:

public class Program
{
    public static void Main(string[] args)
    {
        CreateHostBuilder(args).Build().Run(); // This is where throws error
    }

    public static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            .UseServiceProviderFactory(new AutofacServiceProviderFactory())
            .ConfigureWebHostDefaults(webBuilder =>
            {
                webBuilder.UseStartup<Startup>();
            });
}

Error Detail:

System.NullReferenceException
HResult=0x80004003
Message=Object reference not set to an instance of an object.
Source=Microsoft.Extensions.Hosting
StackTrace:
 at Microsoft.Extensions.Hosting.Internal.Host.<StartAsync>d__9.MoveNext()

If i comment

services.AddHostedService();

Still i get the error. But if i comment all "CreateActivityTask" class and "services.AddHostedService();", then i can run the app.

EmailTaskService can run properly. But if i left "CreateActivityTask" alone (I mean, i commented all "EmailTaskService class etc..), program gets error again.

I can't see any error on CreateActivity class, so i can't find where is the mistake. Could you help me please?